Flexible zeolite lattice

June et al. (12) used TST as an alternative method to investigate Xe diffusion in silicalite. Interactions between the zeolite oxygen atoms and the Xe atoms were modeled with a 6-12 Lennard-Jones function, with potential parameters similar to those used in previous MD simulations (11). Simulations were performed with both a rigid and a flexible zeolite lattice, and those that included flexibility of the zeolite framework employed a harmonic term to describe the motion of the zeolite atoms, with a force constant and bond length data taken from previous simulations (26). 

Other flexible framework calculations of methane diffusion in silicalite have been performed by Catlow et al. (64, 66). A more rigorous potential was used to simulate the motion of the zeolite lattice, developed by Vessal et al. (78), whose parameters were derived by fitting to reproduce the static structural and elastic properties of a-quartz. The guest molecule interactions were taken from the work of Kiselev et al. (79), with methane treated as a flexible polyatomic molecule. Concentrations of 1 and 2 methane molecules per 2 unit cells were considered. Simulations were done with a time step of 1 fs and ran for 120 ps. 

For propane, there is a much better agreement among all the calculations, which considered flexible molecules. The results of Novak etal [14], obtained will both the zeolite lattice and the alkane molecules rigid, show the largest discrepancy. 

The results obtained for the set of molecules studied follow the expected trend, based on the differences of mass and shape of the molecules. Moreover, the results, when compared to the available experimental data, indicate that the simulations can indeed provide a realistic representation of the microscopic process of the diffusion of light hydrocarbons in the pores of zeolites. However, the fact that simulations conducted in very different conditions gave similar results also indicate that a more systematic study is required to establish the influence of the different factors (cluster size, force field, trajectory length, flexibility of the zeolite lattice) in the final results. 

Aromatics. The application of solid acid catalysts provides excellent possibilities to carry out aromatic electrophilic substitutions in an environmentally friendly way. Various zeolites were found by Smith and coworkers to exhibit high activities and selectivities.250 Acetyl nitrate generated in situ from acetic anhydride and HNO3 transforms alkylbenzenes to the corresponding para-nitro derivatives in high yield (92-99%) and with excellent selectivity (79-92%) when applied in the presence of large-pore H-Beta zeolites.251 Lattice flexibility and the coordination of acetyl... 

Haouas, M., Kogelbauer, A. and Prins, R. The effect of flexible lattice aluminum in zeolites during the nitration of aromatics, Stud. Surf. Sci. Catal., 2001, 135, 2113-2120. 

Assuming an ionic-like interaction potential between the framework atoms and cations, Smirnov et al. performed an MD study to assess the cation dynamics in zeolite A. The calculation of the power spectra for Na" and cations at each site revealed that no specific spectral pattern can be attributed to a particular cation position. Vibrations of cations in all sites occur over a frequency region of 30-300 cm In addition, the spectra calculated with a flexible framework showed a substantial coupling between the cationic and lattice degrees of freedom. The results of this work °° have brought into question the assignments proposed for the bands in the far-infrared spectra of cationic forms of zeolites. 

Diffusion and relaxation processes of different nonspherical guest molecules in zeolites are examined by MD simulations with rigid and flexible lattice and molecules. Interrelations between such processes are studied by different tools, e.g. correlation functions. The results show that there exsist cases where a strong mutual influence can be found and others in which there is only small influence. 

P-22 - The effect of flexible lattice aluminum in zeolites during the nitration of aromatics... 

The nitration of toluene with nitric acid and acetic anhydride with zeolite catalysts was studied by means of multi-nuclear solid-state NMR spectroscopy in order to explain the enhanced / ara-selectivity observed with zeolite beta. The reversible transformation of framework aluminum from a tetrahedral into an octahedral environment was revealed by A1 NMR upon interaction of the zeolite with the different components of the nitrating system. The flexibility of the lattice seems to play an important role in the regio-selectivity of nitration catalyzed by zeolites. 

The simulation of catalyst deactivation by coke formation using a 3-dimensional site-bond-site network model is highly attractive, especially for zeolites, as the processes occurring in cavities (also referred to as voids or intersections) and in channels (also referred as necks, capillaries or arcs) can be readily distinguished. This model is flexible and the cormectivity of pores as well as the local homogeneity of the catalyst can be readily altered. Further, a percolation theory is available for site-bond-site models. In the particular case of Bethe lattices, approximated analytical solutions for the percolation probabilities have been derived[7]. 

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